Heroic, brave, or daring deeds or spirit, especially when referring to swordplay or other action-adventure scenarios. The sailor, with great derring-do, leapt from the crow's nest of the ship with his sword in his teeth, pouncing on the pirate captain.
Heroically brave exploits. “Derring” comes from “daring, and “do” is related to “done.” Geoffrey Chaucer originated the phrase in his poem Troilus and Criseyde; it was picked up by Edmund Spenser and again by Sir Walter Scott in Ivanhoe. If you come across it in contemporary speed or writing, you're more than likely to hear it in the longer phrase “deeds of derring do.”
See also: derring